Using Fusion to Propel an Interstellar Probe

The Daedalus interstellar vehicle is designed to be propelled by igniting a series of fusion fuel pellets, 'not vastly different from a conventional internal combustion engine, where small droplets of gasoline are injected into a combustion chamber and ignited.'

Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Headed by the Tau Zero Foundation and British Interplanetary Society, a non-profit group of scientists dedicated to interstellar spaceflight, Icarus is working to develop a spacecraft that can travel to a nearby star.

Richard Obousy, project leader and co-founder of Project Icarus, and primary propulsion lead designer, discusses the propulsion options for an interstellar probe.

On Oct. 4, 1957, the Soviet Union, using an R7-rocket, launched Sputnik into Earth orbit and became the first country in history to launch an artificial satellite.

Less than a decade later, humans were launched into space, and even landed on the moon. Since then we've had many remarkable accomplishments in space — for example, the celebrated Pioneer and Voyager probes, which performed unprecedented scientific research on the gas giants in our solar system.

We also launched the Viking and Venera probes, which landed on Mars and Venus respectively, and there are countless satellites in Earth orbit, including the famous Hubble Space Telescope and the Kepler telescope that have transformed our understanding of the universe with profound scientific discoveries.

These are just a few highlights of humankind's accomplishments in space.

With this stunning portfolio of achievements, it's easy to believe that we've mastered space exploration. However, the reality is that we've barely begun the real adventure.

An Interstellar Civilization?

Possibly the biggest transition for a space faring civilization will be the leap from being an interplanetary civilization, to an interstellar civilization. Interstellar travel involves sending spacecraft, and possibly one day, human beings to other stars. We know that within these star systems exist new planets, some of which may be similar to Earth. But what makes travel to these distant stars so difficult? Why is it that we cannot currently send probes to these planets, just as we do with the planets in our own solar system?

A full analysis of the challenges facing interstellar travel is far beyond the scope of this article; however, the main difficulty lies in the enormous distances that separate the stars. Our closest stellar neighbor, Proxima Centauri, lies just over 4 light years away, meaning that a beam of light traveling at 186,000 miles per second would take over four years to reach it.

Voyager 1, launched in 1977, is the furthest manmade object from Earth, traveling at 10.6 miles per second. Even traveling at this incredible speed, it would take just over 70,000 years to reach the closest star to our solar system.

Recently, NASA began developing Solar Probe plus, which will study our own sun. Through a series of seven gravitational assists with the planet Venus, the probe will reach the extraordinary speed of 125 miles per second. This is a full seven times faster than Voyager 1, which would allow it to make a trip to another star (if that were its objective) in 6,450 years. While this will still be an incredible accomplishment, no currently existing propulsion technology has the capability to fly to another solar system on timescales comparable with a human lifetime.

Project Icarus is a five year theoretical design study of spacecraft that would reach another star within this all-important time restraint. The project was launched in 2009. One of the Terms of Reference of Project Icarus is:

"The spacecraft must reach its stellar destination within as fast a time as possible, not exceeding a century and ideally much sooner."

Clearly, current technology still has some way to come in order to accomplish this goal, based on the figures discussed so far. In fact, based on the distance to the closest star, it appears that we need to accelerate to approximately 5 percent the speed of light, and this is the figure we will focus on for the remainder of the article.

Bigger, Better?

One could be forgiven for just assuming that if we continue to build bigger and bigger chemical rockets, that eventually we'll build one big enough that it could reach 5 percent the speed of light. Interestingly, the laws of physics tell us that this is, in fact, impossible.

The pioneering rocket scientist, Konstantin Tsiolkovsky, developed an equation that predicts how much rocket fuel one would need to reach a given top speed. For chemical rockets, the type that are used today, this equation predicts that to reach 5 percent the speed of light, one would need more chemical rocket fuel than there is matter in the known universe!

Given this sobering result, it's fairly clear that we need to start looking for alternative forms of propulsion if we're ever going to reach the stars on the timescale of a human lifespan.

Two popular technologies that may be able to accomplish this have been explored in some detail. The first are solar sails, which are massive sail-like structures extended in space over many kilometers, which capture the momentum of photons emitted from our own sun to generate acceleration. The second is fusion energy, a form of energy which is known to power our sun, and, as far as we know, all the other stars in the universe.

Fusion typically involves light elements (for example, Hydrogen) being raised to incredibly high temperatures, typically many millions of degrees. When these light nuclei collide, they are able to form new heavier elements and release vast amounts of energy in the process.

To date, our technology has not been able to reach what's known as 'break-even' fusion, where more energy is output from the fusion reactor than was put in to start the reaction. However, there are numerous high profile, and quite a few low profile, fusion research groups across the world focusing a large amount of time and resources into making this a reality. Many believe it is just a matter of a few short decades, perhaps less, until this is done.

What makes fusion particularly appealing for propulsion is the amount of energy that it releases when compared to chemical rocket fuel. A good rule of thumb is that, pound for pound, fusion releases about a million times more energy. Because of this, it is ideally suited for interstellar propulsion.

Fusion Pulse Propulsion

In the 1970's, Project Daedalus demonstrated that with a spacecraft about the size of the Nimitz aircraft carrier, filled mostly with fusion fuel, a top speed of 12 percent the speed of light could be realized. This is truly an incredible speed, and travel to the closest star at this speed would take only 50 years.

One of the many challenges in actually building this technological marvel would be the creation of energies high enough to ignite the fusion reactions which could then be used to propel the spacecraft.

Daedalus used a process known as "pulsed inertial confinement fusion." In this scheme, small pellets of fusion fuel would be injected at a high velocity into a reaction chamber and ignited by high energy electron beams. Conceptually, this is not vastly different from a conventional internal combustion engine, where small droplets of gasoline are injected into a combustion chamber and ignited.

The ignited fusion fuel would reduce the pellet to an expanding plasma radiating from the ignition point. The basic concept of the reaction chamber was to enclose the electromagnetic field of the plasma in a conducting shell. The shell would perform as a shock absorber, which would absorb the momentum of the plasma and transmit it to the vehicle. The process would occur rapidly, over a few microseconds, and the rise and fall in magnetic pressure would be received by the shell as an impulse which set it in motion.

The resulting fusion reaction products in the Daedalus reaction chamber would be channeled axially rearward from the main vehicle by a number of field coils acting as a magnetic nozzle. These ejecta would be responsible for an overall momentum transfer mediated by magnetic fields interacting with the reaction chamber.

The Project Icarus team is currently examining this and several other fusion propulsion schemes, and a decision regarding the main propulsion technology for Project Icarus will be made late in 2012.